我已经尝试了以下代码并且对我来说效果很好..

I have tried the following code and is working fine for me..

    public bool IsUserMemberOfGroup(string userName, string groupName)
    {
        bool ret = false;

        try
        {
            DirectoryEntry localMachine = new DirectoryEntry("WinNT://" + Environment.MachineName);
            DirectoryEntry userGroup = localMachine.Children.Find(groupName, "group");

            object members = userGroup.Invoke("members", null);
            foreach (object groupMember in (IEnumerable)members)
            {
                DirectoryEntry member = new DirectoryEntry(groupMember);
                if (member.Name.Equals(userName, StringComparison.CurrentCultureIgnoreCase))
                {
                    ret = true;
                   break;
                }
            }
        }
        catch (Exception ex)
        {
            ret = false;
        }
        return ret;
    }